MCEN90014 · Materials Engineering
Corrosion & Oxidation
Corrosion & oxidation is the Week 7 electrochemistry topic of MCEN90014 Materials Engineering at the University of Melbourne, where a good alloy fails because it is thermodynamically unstable in service. It sits on the subject's Process→Structure→Property spine as the “environment fights back” chapter, pairing a short-answer side (name the anode, explain a form of attack, justify a defence) with a quantitative side (a Nernst / electrochemical-potential calculation). Get the anode/cathode assignment right and you have unlocked the whole topic.
What this chapter covers
- 01Distinguish oxidation (anode, corrodes) from reduction (cathode, protected) — the OIL RIG rule
- 02See corrosion as a spontaneous, free-energy-lowering electrochemical reaction needing anode + cathode + connection + electrolyte
- 03Rank metals by the EMF series and use the galvanic series for real alloys in a named environment
- 04Apply the standard cell potential ΔV° = V°_cathode − V°_anode and use its positive sign as a check
- 05Correct for real ion concentrations and temperature with the Nernst equation (anode ion over cathode ion)
- 06Identify the forms of attack: galvanic, crevice, pitting, intergranular (sensitisation), stress-corrosion cracking
- 07Explain prevention: material choice/passivation, geometry, coatings, inhibitors, and the area ratio A_cathode/A_anode
- 08Design cathodic protection by a sacrificial anode or an impressed-current system
Cell potential of a zinc–copper couple (Nernst corrected)
- +2Assign anode and cathode: Cu is more noble (+0.340 > −0.763), so Cu is the cathode and Zn is the anode (Zn corrodes). Take subscript 1 = Zn (anode), subscript 2 = Cu (cathode), and n = 2 electrons.
- +2Standard cell potential: ΔV° = V°_cathode − V°_anode = 0.340 − (−0.763) = +1.103 V — positive, so the anode/cathode assignment is correct.
- +2The RT/nF group at 298 K: RT/nF = (8.314 × 298) / (2 × 96 500) = 2477.6 / 193 000 = 0.01284 V.
- +2The log term (anode ion over cathode ion): ln([Zn²⁺]/[Cu²⁺]) = ln(1.0×10⁻² / 0.50) = ln(0.020) = −3.912.
- +2Assemble Nernst: ΔV = ΔV° − (RT/nF)·ln(·) = 1.103 − (0.01284)(−3.912) = 1.103 + 0.0502 = +1.153 V.
Key terms
- Anode
- The electrode where oxidation (loss of electrons, M → Mⁿ⁺ + n e⁻) occurs; it is the metal that corrodes. In a couple it is the more active (more negative V°) metal.
- Cathode
- The electrode where reduction (gain of electrons, e.g. O₂ + 2H₂O + 4e⁻ → 4OH⁻) occurs; it is protected from corrosion. In a couple it is the more noble (more positive V°) metal.
- EMF series
- A ranking of pure metals by their standard electrode potential V° (volts vs the standard hydrogen electrode, 25 °C, 1 M ions). More negative V° = more active = more likely to be the anode.
- Galvanic series
- A ranking of real alloys by nobility in a specific environment (classically seawater). It is the practical predictor of a couple and can differ from the idealised EMF series.
- Standard cell potential (ΔV°)
- ΔV° = V°_cathode − V°_anode, in volts. It must be positive for the spontaneous couple; a negative result means the assumed anode and cathode are reversed.
- Nernst equation
- ΔV = (V°₂ − V°₁) − (RT/nF)·ln([M₁ⁿ⁺]/[M₂ⁿ⁺]), which corrects ΔV° for real ion concentrations and temperature. Subscript 1 = anode, 2 = cathode; R = 8.314 J K⁻¹ mol⁻¹, F = 96 500 C mol⁻¹, T in kelvin, RT/F ≈ 0.0257 V at 25 °C.
- Sensitisation
- Precipitation of Cr₂₃C₆ at grain boundaries of stainless steel held at 500–800 °C (e.g. during welding), depleting nearby chromium and causing intergranular corrosion (weld decay). Countered with low-carbon (304L/316L) or Ti/Nb-stabilised grades.
- Cathodic protection
- Forcing a structure to be the cathode so it cannot oxidise — either by a sacrificial anode (a more active metal such as Mg/Zn that corrodes instead) or by an impressed-current system driven by an external DC supply.
Corrosion & Oxidation FAQ
How do I decide which metal in a couple corrodes?
Read the standard electrode potentials: the metal with the more negative V° is the more active one, so it becomes the anode and corrodes, while the more noble (more positive V°) metal is the cathode and is protected. Confirm it with ΔV° = V°_cathode − V°_anode, which must come out positive for the spontaneous couple — if it is negative you have the anode and cathode the wrong way round. Real service problems use the galvanic series (for alloys in that environment) rather than the pure-metal EMF series.
Why does zinc protect steel but tin can make it corrode faster?
It comes down to the direction of the couple. Zinc is more active than iron, so on galvanised steel the zinc is the anode and sacrificially protects the steel even where the coating is scratched. Tin is more noble than iron, so a scratch in tinplate turns the exposed steel into a small anode under a large tin cathode; the large area ratio A_cathode/A_anode drives fast, deep perforation. Tin only protects as an intact barrier.
Can AI help me with corrosion and oxidation in MCEN90014?
Yes — Sia is an AI tutor that can explain the concepts step by step: it can walk you through assigning the anode and cathode, show how to set up the Nernst equation with the anode ion on top of the log ratio, or help you tell crevice from pitting corrosion. It is a study aid for understanding the method, not a source of ready-made answers, and it will not sit an assessment or guarantee a grade. Always follow the University of Melbourne's academic-integrity and generative-AI rules for MCEN90014 and confirm what is permitted on Canvas.
Studying with AI? Sia — free AI mechanical engineering tutor works through MCEN90014 step by step.
Exam move
Anchor the whole topic on one decision: which electrode is the anode. Practise reading it off the electrode potentials (more negative V° = anode = corrodes), then writing ΔV° = V°_cathode − V°_anode and checking it is positive before you go near the Nernst correction. When you apply Nernst, drill the three habitual slips — keep the anode ion on top of the log ratio, put temperature in kelvin, and use the right number of electrons n — and end with a positive ΔV as your audit. Because a formula sheet (with its own electrode-potential table) is supplied in the final exam, spend your practice time choosing the right relation and substituting cleanly with SI units, not on memorising equations. Learn to name the five forms of attack (galvanic, crevice, pitting, intergranular, stress-corrosion cracking) with a one-line mechanism and a matching defence, and be able to explain cathodic protection by a sacrificial anode. The final exam is 10 questions of 10 marks each (100 marks, all compulsory) worth 50% of the subject with an exam hurdle, so treat every question as equally weighted — about a tenth of your time each — and confirm the exam duration on the timetable in Canvas.
